Various instruments for analyzing the characteristics of materials rely on sensors for at least a portion of their measurement operations, with these sensors being chilled to low temperatures to enhance measurement accuracy (e.g., by decreasing electronic “noise”). As an example, electron microscopes often include an X-ray detector (such as a silicon sensor) mounted at the end of an elongated probe or other mount, often called a “cold finger,” which is situated next to a specimen to be analyzed. The cold finger is chilled to cryogenic (ultralow) temperatures, usually by a Dewar system utilizing liquid nitrogen coolant, though some systems use a standard refrigeration cycle for cooling (i.e., evaporative cooling). Additionally, one provider (Thermo Electron, Madison, Wis., USA) has long provided thermoelectric (Peltier) cooling of detectors. During operation, as the specimen is bombarded by electrons from the microscope's electron beam, it emits X-rays which are picked up by the detector. The detector measurements can be processed to provide information regarding the specimen's material and other characteristics.
These arrangements suffer from the unfortunate disadvantage that while cooling of the detector enhances measurement quality, cooling also increases the possibility that the detector will be fouled (and its measurements skewed) owing to water/oil condensation, and ice formation, on the cooled detector. Moisture and oil are often present in the analysis chamber wherein the specimen and detector are located, with the oil originating from the vacuum pumping system. While they can be diminished by steps such as evacuating the analysis chamber so the specimen and detector are in vacuum (a common step), ice and oil condensates still tend to collect on the detector owing to factors such as residual gas within the analysis chamber and moisture release from the specimen. Some detectors and mounts are partially insulated from the analysis chamber by a surrounding shell about the mount and/or a window between the chamber and the detector; however, even these arrangements tend to accumulate ice and oil on the shell and/or window. Additionally, while windows help protect detectors from contamination, they can also block lower-energy emissions that could otherwise be usefully detected by the detector.
As discussed in U.S. Pat. Nos. 4,931,650 and 5,274,237, the foregoing difficulties have led to the development of a variety of corrective devices and methodologies. Both patents describe the use of periodic warm-up cycles wherein the mount and detector are allowed to warm up to drive off water. U.S. Pat. No. 4,931,650 assists such a procedure by incorporating a resistive heater for warming the detector, and U.S. Pat. No. 5,274,237 has a portion of the analysis chamber about the detector at a cooler temperature so that the bulk of any ice will form away from the detector. However, it would be useful to have further arrangements available for avoiding detector ice contamination in electron microscopes and other analytical instruments.